The present invention in general concerns an improved metal detector, and in particular concerns an induction-type metal detection system with a particular detector head coil configuration suitable to be adapted for use with single or multiple head embodiments. Supporting electronic circuitry for use with either single or multiple head embodiments forms additional aspects of the present invention.
Heretofore, numerous coil configurations and detector head arrangements have been known for metal detector devices. One generally well-known basic approach to transmit/receive coils (i.e. primary/secondary coils) involves a three coil arrangement, wherein one transmit coil is operatively associated with two receive coils which are wired in opposition to one another. Basic detection principles of such three coil configurations are also well known to those of ordinary skill in the art.
Examples of such three-coil detector head configurations may be found in McDaniel (U.S. Pat. Nos. 3,882,374; see FIG. 2 and column 2, lines 49-53); Mansfield (U.S. Pat. No. 3,652,928; see the Figure and column 2, lines 20-24); Binard (U.S. Pat. No. 4,552,134; see FIG. 3 and column 3, lines 20-23); Bondarenko et al. (U.S. Pat. No. 3,942,105; see FIG. 2 and column 2, lines 43-45); and Demopoulos et al. (U.S. Pat. No. 3,002,262; see FIG. 1 and column 1, lines 17-24). In general, the disclosures of such patents, and particularly with respect to their electronic circuitry for detection of metal objects and the operating principles thereof, are incorporated herein by reference.
All of the foregoing patents are generally directed to a single detector head arrangement, whether the coils thereof are situated in a co-planar arrangement (i.e. common plane) or not. For example, FIG. 3 of Binard (U.S. Pat. No. 4,552,134) clearly illustrates that radiating coil 6 thereof is in a different plane from the pair of receiving coils 7. As clearly illustrated by such FIG. 3 of Binard, non-planar arrangements can be bulky and require additional (perhaps critical) space.
McDaniel (U.S. Pat. No. 3,882,374), while showing a co-planar arrangement of three coils in FIG. 2 thereof, illustrates another non-co-planar arrangement in its Prior Art FIG. 1. In such Prior Art figure, five coils (one transmit and four receive) are situated in two separate planes. In both embodiments, (the Prior Art FIG. 1 and his inventive FIG. 2), McDaniel provides receiving coils which are at least in part disposed outside the periphery of his transmitting coil.
In addition, prior single detector head systems are in general not well suited nor readily adapted for use in multiple head configurations so as to increase the scanning area for a metal detection system. For example, a single head unit consisting of a three foot by three foot detector head only covers a nine square foot area at a time. If such detector head were repeated in a one by eight array, with each head still measuring three feet by three feet, then such a multiple head detector could cover a 72 square feet scanning area at a time providing tremendous advantages over the single head unit, particularly whenever large scale scanning operations are undertaken.
However, as alluded to above, a single head detector using for example a tuned coil concept (well known to those of ordinary skill in the art) does not lend itself to such multiple head use. In general, with a tuned coil concept detector head, once the coil is tuned it will stay tuned until some form of metallic material affects the coil and the coil becomes detuned which causes a voltage rise in secondary coils thereof. Such a voltage rise is indicative of the presence of metallic material within the field of the detector head. One particular short-coming or disadvantage in attempting to apply the tuned coil technology to multiple detector heads is that each detector head would necessarily have to be tuned to a different frequency in order to avoid interference with adjacent detector heads. If eight heads were used in a single detection system, then a relatively large bandwidth would be required.
Hence, such prior technology in general is undesirable for (or in some cases practically incapable of use in) multiple head metal detector systems.